Conventionally, as a method of synchronizing data between information processing apparatuses, there is a method of difference synchronization that performs synchronization by using only the difference information in order to reduce the data transfer amount on a network. Japanese Patent Laid-Open No. 2007-115007 proposes a technique of updating data of one information apparatus by using data of another information processing apparatus, if the data of the one apparatus is deleted due to restore processing when past data is restored and recovered while performing difference synchronization. It also proposes, for a case where synchronization is performed between a server and a client, a synchronization system that only temporarily uses data of the client to keep data of the server constantly as the master. In Japanese Patent Laid-Open No. 2013-250759, there is proposed a system that performs synchronization while holding temporary data differences of a client even when it is disconnected from the network.